1 Effects of rapid thermal annealing on the morphology and electrical properties of ZnO/In films Tae Young Ma, Dae Keun Shim Department of Electrical Engineering & Research Institute of Industrial Technology, Gyeongsang National University, 900 Gazwa-dong, Chinju, Gyeongnam , South Korea Thin Solid Films 410 (2002) 8–13 學生 : 劉璟霈 指導教授 : 林克默老師 09/03/20 文獻回顧

2 Outline Abstract Introduction Experimental procedure Results and discussion Conclusions

3 Abstract Zinc oxide (ZnO) films were prepared by ultrasonic spray pyrolysis on indium (In) films deposited by evaporation and subsequently subjected to rapid thermal annealing (RTA) in air or vacuum. Auger electron spectroscopy (AES) was carried out to determine the distribution of indium atoms in the ZnO films. The resistivity of the ZnO on In (ZnO/In) films decreased to 2 ×10 -3 Ω cm by diffusion of the In. The results of depth profiling by AES showed a hump of In atoms around ZnO/In interface after RTA at 800 ℃,which disappeared on RTA at 1000 ℃.

4 Introduction ZnO films, in particular, are attracting more attention as transparent conductive films and gas sensors because of their amenability to doping, their low cost, non-toxicity and stability in hydrogen atmospheres Recently, several researchers have reported that ZnO films can be easily grown by ultrasonic spray pyrolysis (USP). The USP technique is considered to be a very useful method due to the simplicity of the apparatus and the low cost of the raw materials used.

5 In USP, zinc acetate and indium chloride diluted in an alcoholic solution have previously been used as sources for ZnO:In films. However, we found that the preparation of ZnO:In films from In-containing solution is not effective from the viewpoint of reproducibility and uniformity of the dopant distribution. In this paper, we propose a new process for the preparation of ZnO:In films. The merit of this new method is in the simplicity of the process. We can easily modulate the electrical and morphological properties of ZnO/In films by controlling the RTA treatment conditions.

6 Experimental procedure Silicon dioxide-coated Si wafers were used as substrates. A ;10-nm-thick In film was deposited on the substrates by a thermal evaporation method. A ;300-nm-thick ZnO film was subsequently grown on the In film by the USP method. In the USP process, zinc acetate diluted in methanol was selected as the zinc precursor at a concentration of 0.03 mol1 -1. Aerosols produced by pulverizing the solution are conveyed to the reactor by nitrogen gas at a flow rate of 1 1min -1. The substrate temperature was 230 ℃. We annealed the ZnO/In film in a RTA apparatus(MILA-3000, ULVAC) to diffuse In atoms into the ZnO film. The annealing temperatures were 600, 800 and 1000 ℃ and the annealing time varied between 10 s and 4 min.

7 Results and discussion Fig. 1 shows the XRD spectra of ZnO films on SiO2 RTA- treated at various temperatures. The XRD patterns show that as-deposited ZnO films have a poor polycrystalline structure. RTA at 800 ℃ for 30 s leads to an increase in preferred orientation, with the (002) axis perpendicular to the substrate. Fig. 1. XRD spectra of ZnO films annealed at: (a) 800; (b) 1000 ℃ for 30 s in air; and (c) as-deposited.

8 The XRD peaks of these ZnO/In films are shown in Fig. 2b. From the XRD peaks, it is believed that the lack of oxygen and a fast diffusion process in vacuum deter the In-doped ZnO films from recrystallizing. The peak intensities of the (100) plane parallel to the substrate and of the (002) plane normal to the substrate are notable in Fig. 2a. It seems that annealing temperature higher than 800 ℃ for 30 s deteriorates the crystallites of the ZnO/In films.

9 The highest peaks of In2O3 powder (222 and 400 planes) are found at 2 θ =29.88 and However, oxygen in the ZnO film is not sufficient to form In2O3 film and the RTA time is too short for oxidation of In. We believe that In2O3 cannot be formed during this RTA treatment. The mean grain size of the film annealed at 1000 ℃ for 30s was found to be 220 nm, with standard deviation of 78 nm.

10 Fig. 3. SEM images of ZnO films annealed at: (a) room temperature;(b) 800; and (c) 1000 ℃ for 30 s in air. Fig. 3 shows the SEM images of the ZnO films corresponding to Fig. 1. The as-grown ZnO film appears to be an amorphous structure.

11 The SEM images of the ZnO/In films rapidly annealed at various temperatures are shown in Fig. 4. The film annealed at 600 ℃ (Fig. 4a) shows fine grains. The grain size increases with increasing RTA temperature (Fig. 4b,c). Fig. 4. SEM images of ZnO/In films annealed at: (a) 600; (b) 800; and (c) 1000 ℃ for 30 s in air. Comparing Fig. 3 and Fig. 4, it is evident that In diffusion into the ZnO film deters grains from growing in a hexagonal shape, which roughens the surface morphology of the films.

12 Fig. 5 shows SEM images of the ZnO/In films annealed in vacuum at 800 and 1000 ℃ for 30 s. No clear grain boundaries are evident in Fig. 5, which is consistent with the XRD results of Fig. 2b. Fig. 5. SEM images of ZnO/In films annealed at: (a) 800; and (b) 1000 ℃ for 30 s in vacuum.

13 Fig. 6. Resistivity variation of ZnO/In films with RTA time. The samples were annealed at (a) 600 ( ▲ ), 800 ( ■ ) and 1000 ℃ ( ● ) in air; and (b) 800 ( ■ ) and 1000 ℃ ( ● ) in vacuum.

14 Fig. 7. Auger depth profiles for ZnO/In films annealed in vacuum at (a) 800 ℃ for 10 s; (b) 800 ℃ for 30 s; and (c) 1000 ℃ for 10 s. Fig. 7a shows that a hump of In atoms is found at the ZnO/SiO2 interface after annealing at 800 ℃ for 10 s; Fig. 7b shows that the hump diminishes with increasing annealing time. Fig. 7c shows that the hump of In atoms has disappeared on RTA at 1000 ℃ for 10 s. Into the vacuum, which increases the resistivity of the films annealed at 1000 ℃.

15 Fig. 8. AES spectrum of a ZnO film annealed at 1000 ℃ for 10 s. The peak at approximately keV in Fig. 8 may be discarded as noise. If this peak were not from noise, but from Si, another peak should be detected at approximately 1.6 keV. However, no notable peak was found at this value. Therefore, the Si concentration in the ZnO region in Fig. 7, mostly obtained from the peak at keV, is negligible.

16 Conclusions Zinc oxide films were prepared by USP on ;10-nm thick indium films, followed by RTA in air or vacuum. However, the XRD peak of the (100) and (002) directions became pronounced after the RTA treatment in air. Recrystallization of the Indoped ZnO films in vacuum was suppressed by a lack of oxygen and a fast diffusion process. The In diffusion into ZnO film deters grains from growing in a hexagonal shape, which roughens the surface of the films. Anneal at 1000 ℃ for 10s and the resistivity to 2 ×10 -3 Ω cm. We can easily change the surface morphology and resistivity of ZnO films by In doping with the RTA process, which will widen the applicability of the ZnO films.

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